CCAMLR

We revised the method of Candy et al. (2012) to estimate the ageing error matrix (misclassification matrix) to address some issues regarding true ages not being the mode at the extremes of the matrix and a lack of smoothness in the probabilities for ages above 25 years. Positive errors now aggregate into the plus group, negative errors are truncated below the minimum age, and fish with true ages greater than the plus group are incorporated into the final row of the misclassification matrix. We expanded the reference collection to include an additional 50 otoliths with a mean age above 25 years. The revised method was applied to the expanded reference collection to re-estimate the misclassification matrix.We assessed misclassification matrices obtained using the revised Candy method along with two other methods of accounting for ageing error, approximation by a Gamma distribution and a constant coefficient of variation (c.v.) of 0.1. We compared the misclassification matrices with the observed age determinations from the expanded reference collection. We then used estimates of spawning stock biomass (SSB) and year class strength (YCS) from the CASAL assessment of the HIMI fishery to evaluate the effects of these three methods along with assuming no ageing error. The Gamma and revised Candy methods both provided satisfactory representations of the observed age determinations. The constant c.v. method provided a poor representation of the observed ages, underestimating the variability at the younger age classes and over estimating the variability at the older age classes. Estimates of SSB were similar for the revised Candy and Gamma methods. The constant c.v. method and assuming no ageing error produced similar estimates of SSB that were consistently lower than those from the revised Candy and Gamma methods. Not accounting for ageing error gave the most precise and least variable estimates of YCS while the Gamma method gave the least precise and most variable estimates. Estimates of YCS from the constant c.v. method and the revised Candy method were between the other two methods in terms of year to year variability and precision. The poor agreement of the constant c.v. method with observed ages and the similarity of SSB estimates to those obtained assuming no ageing error would suggest it is not appropriate for use in the HIMI assessment and we would caution others to assess its impact before applying it to their assessments. Estimates of SSB from the Gamma method were consistent with the revised Candy method, suggesting this method may be of use where there is insufficient data to estimate ageing error directly.

The Australian Antarctic Division (AAD) has recently received funding to continue an ageing program for the purposes of routine processing of Patagonian toothfish (Dissostichus eleginoides) otoliths from the Heard Island and McDonald Islands fishery. With the reinitiation of this program a number of methodological refinements have been made to those outlined by Nowara et al. (2009), particularly in relation to the laboratory processing of otoliths and quality control in the assignment of fish age. These changes have contributed to increased efficiency within the program, with a greater capacity for ageing and at a lower cost per otolith. Greatest efficiency gains have been made in the laboratory, with otoliths now set in smaller resin blocks that take much less time to section, and in quality control and reporting procedures, which are now largely automated using R.

A key part of quality assurance in any ageing program is to minimise the effect that a reader has in assigning ages.  In the AAD program, two readers look at all otoliths, which has proved useful in helping to identify  otoliths that may be  interpreted differently by different readers, so they can be flagged, re-read, and consensus reached on the most likely age estimate.  Using the otolith weight/age relationship has also been useful for identifying otoliths that may have been aged with good precision between the two readers, but may represent systematic error in interpretation (e.g. both readers may have counted all sub-banding for a particular fish).

This year’s stock assessment includes new age data from the 2012, 2013 and 2014 RSTSs, as well as from the commercial fishery in 2013. The recent focus by AAD to age more large fish from commercial samples has substantially increased the information available on older age classes (i.e. fish greater than 20 years) available for use in the current assessment (Ziegler et al., 2014).
 

The annual Random Stratified Trawl Survey was undertaken in Division 58.5.2 in the vicinity of Heard Island during June 2014. This paper provides a preliminary assessment of mackerel icefish population structure, abundance and yield for the area of Division 58.5.2 to the west of 79° 20’ E using standard CCAMLR methods. A pattern of multiple year classes being present simultaneously in the population, which was first detected in 2011, has persisted. The large cohort observed in recent surveys was still numerically dominant, however there is evidence that recruitment to the 0+-3+ cohorts has taken place. Given the expectation that the current 4+ and 5+ cohorts are fully exploited, only the 1+-3+ cohorts were projected. These projections indicate that catches of 309t in the 2014/15 season and 275t in the 2015/16 season satisfies the CCAMLR decision rules.

Patagonian toothfish have been tagged during commercial fishing and research activities adjacent to Heard Island and the McDonald Islands (HIMI, CCAMLR Division 58.5.2) since 1998, with 27,679 toothfish tagged and released, and 4925 recaptures. The data derived from this program has been valuable in estimating key population parameters for toothfish in this region, and have been used in the 2014 stock assessment (Ziegler, et al, 2014) in various ways including:

•  local abundance estimators;
•  natural mortality;
•  movement rates;
•  growth;
•  stock structure; and
•  estimation of q of the trawl surveys;

However, due to the spatial pattern of releases, toothfish movement, and recapture effort, it is unlikely that these data can currently be used in an unbiased manner to estimate stock abundance. Developing spatially explicit models that account for fleet dynamics, and phenotypic variability in post-release behaviour, remains an issue for all assessments incorporating tagging data and, in particular, in Division 58.5.2. Plans to develop such spatial models, as well as increasing tagging rates and attempting to spread the release and recapture effort, are likely to rapidly improve the utility of this data for future assessments.
 

The Kerguelen Plateau supports the largest fishery for Patagonian toothfish (Dissostichus eleginoides) in the CCAMLR area, with landings of  >5000 t.yr^-1 from the French EEZ (Division 58.5.1) around îles Kerguelen and 2730 t.yr^-1 from the Australian EEZ around Heard and McDonald Islands (HIMI, Division 58.5.2). This paper provides a synthesis of the spatial distribution of the Patagonian toothfish drawn from biological data collected in the HIMI area (Division 58.5.2) on more than 500,000 fish caught since 1997 during fishery operations and research surveys. Statistical analyses were used to quantify the effect of bathymetry in structuring the spatial distribution of different length classes and sex composition, when controlling for gear selectivity, year and sex effects. Fish length increased with depths suggesting spatial segregation of life stages. Spatial predictions showed that small, immature, fish were mostly found over the Plateau (<400m) and banks tops (i.e. Shell Bank), whereas larger fish inhabit deeper waters (>1500m) especially on the eastern part of the Plateau, where the continental slope is steep. Being slightly biased towards females, sex ratio was also influenced by fishing depth. Proportion of females increased at shallower (<500m) and deeper (>1200m) depths, whereas a sex ratio of 0.6 was found in between. The recent discovery of extensive areas of spawning activity in the Australian EEZ revealed the importance of the western margin of the Plateau as spawning habitat. These results allow us to further refine hypotheses about the spatial segregation of life-stages and sex in the HIMI part of the Kerguelen Plateau. The distribution and ecology of the early life history stages, up to 2-3 years old, and the implications of sexual and phenotypic variability in growth and movement stand out as the priority for future research into the biology and management of Patagonian toothfish in this region.

The annual random stratified trawl survey (RSTS) at HIMI was conducted during June of 2014, with the completion of 158 stations. An additional five stations were completed at Shell Bank, which had not been sampled since 2005. The catches of C. gunnari were less than half of those in 2013, but catches of both D. eleginoides and C. gunnari were higher than the long-term average (2006-2013). Catches of Channichthys rhinoceratus and Lepidonotothen squamifrons were higher than average. Skate catches in 2014 were slightly higher than the average RSTS take. Catches at Shell Bank consisted mostly of L. squamifrons and a small quantity of C. rhinoceratus. There were no C. gunnari  caught in this stratum.

The catch of invertebrates in the 2014 survey was 3.4 t, with the most abundant taxa being the poriferan sponges. Catches were half the long-term average.

Biological measurements were taken for 8,500 fish and lengths of a total of 18,000 fish were recorded. Otoliths were collected from 617 D. eleginoides, and 245 were tagged. Additionally, 124 sets of otoliths were collected from L. squamifrons.
 

This paper reports on the trial of daytime fishing during the 1-15 April pre-season extension, in the Patagonian toothfish longline fishery in statistical division 58.5.2.

During the 1-15 April pre-season extension two vessels fished, and all setting was carried out during nautical dusk or night. No seabirds were caught.

Any fishing that occurs in the 1-14 November 2014 post-season extension period, and the planned in the April 2015 extension will be reported to WG-FSA in 2015.
 

During the 2012 CCAMLR toothfish ageing workshop, otoliths from 31 Antarctic toothfish were exchanged between Russia and New Zealand ageing programmes to compare consistency in ages estimated by different readers and using different methods. Both “break and burn” and “bake and embed” techniques were used to prepare otoliths and each were read by an experienced reader from each program. The resulting four-way comparison enabled differences in preparation method to be distinguished from differences in interpretation of otolith banding patterns. Results suggest broad agreement in ages determined by each reader and with each method. However, there were enough inconsistencies in preparation technique and in interpretation of the break and burn preparation method to warrant further coordination and comparisons before merging data. Bake and embed preparations were consistently interpreted by both readers. This experiment highlights the importance of monitoring and comparing ageing protocols within and between fish ageing programmes. The criteria for determining similar age interpretation used here (mean paired age difference statistically equal to zero, overall CV less than 10%, and slope of regression across ages equal to 1) were useful diagnostics in interpreting age comparisons.